This is just a semi-regular reminder that arranging is composition.
Aside from creating a melody, every aspect of composing is exactly mirrored in arranging. Key selection, shaping of form, orchestration choices, methods of development and creative adapting of the melody: these are all present in good arranging as well as composition.
Indeed, I think that arranging is a far better place to start for most composition students and all music literacy students than composition. I had no business writing a Romantic sonata allegro in sophomore theory, because my understanding of creating music was entirely harmonic, thanks to the focus of freshman theory. My melody was surely substandard – and why shouldn’t it have been? Crafting a good melody is a distinct and separate skill that is never covered in collegiate music theory.*
I would have been far better receiving a melody and arranging in sonata allegro form. Indeed, at every step of the way, music students will succeed better if they are arranging an outstanding melody using their theory knowledge, rather than trying to create a new melody that will be suitable for their theoretical needs.
These days, I compose and arrange regularly, and I feel the kinship in my very soul. Remind yourself: arranging is composition. It’s not in any way inferior to composition; it just uses someone else’s melody.
* If you’re looking for the missing training in understanding good melodies, look no further than Alice Parker’s wonderful book, The Anatomy of Melody.